1. Field of Invention
The present invention relates to a permanent magnet, more particularly an Nd--Fe--B sintered magnet, and to a method for producing the same.
ln the Nd--Fe--B magnets there are melt-quenched magnets and sintered magnets. Essentially, the melt-quenched magnet is magnetically isotropic. There is a method under proposal for rendering the melt-quenched magnet anisotropic, which resides in crushing a strip obtained by melt-quenching to produce a powder, hot-pressing and then die-upsetting the powder. This method, however, has not yet been carried out industrially, since the production steps are complicated.
2. Description of Related Arts
Nd--Fe--B sintered magnet has been developed by the present inventor et al. It has outstanding characteristics in that it exhihits excellent magnetic property in terms of 50 MGOe of maximum energy product (BH)max in a laboratory scale and 40 MGOe even in a mass production scale; and, the cost of raw materials is remarkably cheaper than those of the rare-earth cobalt magnet, since the main components are Fe and B, and Nd (neodymium) and Pr (praseodymium), all inexpensive elements, which are relatively abundant in the rare-earth elements. Representative patents of the Nd--Fe--B sintered magnet are Japanese Unexamined Patent Publication No. 59-89401, Japanese Unexamined Patent Publication No. 59-46008 (Japanese Examined Patent Publication No. 61-34242, Japanese Patent No. 14316170), Japanese Unexamined Patent Publication No. 59-17003), USP No.4597938 and European Patent No. EP-A-0101552. As for academic papers, there is "New Material for permanent magnets on a base of Nd and Fe (invited)", M. Sagawa et al, J. Appl. Phys., 55, No.6, Part II, p 2083/2087 (March, 1984).
The present applicant filed a patent application U.S. Ser. No. 07/321,183 directed to the following magnets (A) and (B), below. The description of prior art in this earlier patent application is referred hereinto.
The present inventor researched and discovered the following. That is, in a V-added Nd--Fe--B magnet having a specified composition, the NdFe.sub.4 B.sub.4 phase (B rich phase) is suppressed to the minimum amount, and a compound phase other than the NdFe.sub.4 B.sub.4 phase, i.e., a V--Fe--B compound phase, whose presence is heretofore unknown, is formed and replaces the NdFe.sub.4 B.sub.4 phase, i.e., B rich phase. An absolute value of the coercive force (iHc) is exceedingly enhanced and the stability at high temperature is improved due to the functions of both V--Fe--B compound phase and the particular composition. In addition, the corrosion resistance of the Nd--Fe--B sintered magnet is greatly improved by the formation of the V--Fe--B compound phase and disappearance or decrease of the NdFe.sub.4 B.sub.4 phase.
Based on such a discovery, the invention of the earlier application was made.
(A) An R--Fe--B sintered magnet has a temperature-coefficient of coercive force (iHc) of 0.5%/.degree. C. or more, and such a composition that R=11-18 at % (R is one or more rare-earth elements excluding Dy, with the proviso that 80 at % (Nd+Pr)/R 100 at %), B=6-12 at %, and the balance is Fe and Co (with the proviso that Co is 25 at % or less relative to the total of Co and Fe (including 0% of Co)) and impurities. Also it is characterized in that B, in excess of a stoichiometric composition of a R.sub.2 Fe.sub.14 B.sub.4 compound-phase, essentially does not form a RFe.sub.4 B.sub.4 -compound minority phase but forms a finely dispersed V--T--B compound minority phase (T is Fe, and in a case containing Co, T is Fe and Co), and, further, the magnet exhibits 20 MGOe or more of maximum energy product and 15 kOe or more of coercive force (iHc).
(B) An R--Fe--B sintered magnet has a temperature-coefficient of coercive force (iHc) of 0.5%/.degree.C. or more and such a composition that R=11-18 at % (R is rare-earth elements, R.sub.1 =Nd+Pr, R.sub.2 =Dy, with the proviso that 80 at %.ltoreq.(R.sub.1 +R.sub.2)/R.ltoreq.100 at %), 0.ltoreq.R.sub.2 .ltoreq.4 at %, B=6-12 at %, and the balance is Fe aad Co (with the proviso that Co is 25 at % or less relative to the total of Co and Fe (including 0% of Co)) and impurities. Also it is characterized in that B, in excess of a stoichiometric composition of R.sub.2 Fe.sub.14 B compound-phase, essentially does not form a RFe.sub.4 B.sub.4 -compound minority phase but forms a finely dispersed V--T--B compound minority phase (T is Fe, and in a case containing Co, T is Fe and Co), and, further, the magnet exhibits 20 MGOe or more of maximum energy product and 15+3x (kOe) of coercive force (iHc) (x is Dy content (at %), with the proviso that when 15+3x (kOe) is 21 kOe or more, the coercive force is 21 kOe or more).